I have a different answer.
You are asking about shrinking testes as far as I can tell, not about your implant, so I would say that you need not only an exam, but testosterone levels checked, and an endocrinologic exam to see if there are other reasons for this.
Shrinkage, if from the implant surgery is not reversible, but you need to rule out other causes.
There is also no reason to believe, if this was from the surgery, that the other testicle will necessarily deteriorate, but other problems need to b ruled out.
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